Springer Online Journal Archives 1860-2000
Abstract A partial cDNA which codes for the β-subunit of a plastidic chaperonin 60 (cpn60-β) from rye (Secale cereale L.) leaves was identified and sequenced, except for 46 amino acids of the N-terminus of the mature protein and the transit sequence. This is the first cpn60-β sequence determined for a monocotyledonous plant. Specific antibodies against cpn60-β were affinity-purified from an antiserum raised against the total soluble protein fraction of ribosome-deficient plastids. The localization of cpn60-β in chloroplasts or non-green plastids was confirmed by immunodetection in Percoll gradient-purified organelles. The expression and occurrence of cpn60-β was analysed by immunoblotting with the specific antibodies and Northern hybridization. The cpn60-β protein was constitutively expressed in various green and non-green tissues. It was evenly distributed along the major part of a rye leaf, while highest transcript levels occurred in the youngest and oldest leaf sections. The expression of the cpn60-β protein was not enhanced by a heat-shock treatment at 42 °C. The cpn60-β transcript and protein were more strongly expressed in various non-green, for instance etiolated, 70S-ribosome-deficient 32 °C-grown, or herbicide-bleached tissues, than in green leaves of rye. A rapid increase in the cpn60-β transcript level was also observed when green leaves were transferred from light to darkness while the protein level was not affected. The dark-induced increase in the cpn60-β transcript was totally suppressed in the presence of 2% sucrose. Inhibitor treatments suggested that the change in cpn60-β transcript level was not related to changes of the ATP supply of the tissue. While the large subunit of the photosynthetic protein ribulose-1,5-bisphosphate carboxylase was largely degraded during ripening of tomato fruits, high levels of cpn60-β were detected in tomato chromoplasts and in the yellow flower petals of Narcissus. Low levels of cpn60-β were detected in root tissue.
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